CA DE RECTO ANATOMIA

ANATOMIA   The anatomy of the rectum can be very confusing as there are differing definitions of the relevant landmarks. In the upper portion of the rectum there are changes both in the musculature of the large bowel and in the relationship to the peritoneal covering that roughly coincide. In the lower portion of the rectum the mucosal changes occur at roughly the same location as the anal sphincter. The anatomy of the rectum is usually divided into three portions (Fig. 39.13.1). The lower rectum is the area approximately from 3 to 6 cm from the anal verge. The midrectum goes from 5 to 6 to 8 to 10 cm, and the upper rectum extends approximately from 8 to 10 to 12 to 15 cm from the anal verge, although the retroperitoneal portion of the large bowel often reaches its upper limit approximately 12 cm from the anal verge. In some patients, especially elderly females, the peritonealized portion of the large bowel can be located much lower than these definitions. The determination of the location of the boundary between rectum and sigmoid colon is important in defining adjuvant therapy, with the rectum usually being operationally defined as that area of the large bowel that is at least partially retroperitoneal. Externally, the upper extent of the rectum can be identified where the tenia spread to form a longitudinal coat of muscle. The upper third of the rectum is surrounded by peritoneum on its anterior and lateral surfaces but is retroperitoneal posteriorly without any serosal covering. At the rectovesical or rectouterine pouch, the rectum becomes completely extra(retro)peritoneal. The rectum follows the curve of the sacrum in its lower twothirds. It enters the anal canal at the level of the levator ani. The anorectal ring is at the level of the puborectalis sling portion of the levator muscles. The location of a rectal tumor is most commonly indicated by the distance between the anal verge, dentate (pectinate or mucocutaneous) line, or anorectal ring and the lower edge of the tumor. These points of reference are all different for different individuals. Also, these measurements differ depending on the method of measurement. This can be important clinically, as the measurement from a flexible endoscopy can substantially overestimate the distance to the tumor from the anal verge or other landmark. The distance from the anal sphincter musculature is clinically of more importance than the distance from the anal verge, as it has implications for the ability to perform sphincter-sparing surgery. The lack of a peritoneal covering most of the rectum is a major reason for the higher risk for local failure after primary surgical management than for colon cancer. The mesorectum is usually used as the structure to define the extent of a total mesorectal excision, with most of the perirectal fatty tissue and perirectal lymph nodes contained within its boundaries. P.1286 Lymphatic Drainage The lymphatic drainage of the upper rectum follows the course of the superior hemorrhoidal artery toward the inferior mesenteric artery. Lymph nodes that are above the midrectum and therefore drain along the superior hemorrhoidal artery are often part of the mesentery that is removed during resections of the intraperitoneal portion of the colon. Lesions that arise in the rectum below approximately 6 cm are in a region of the rectum that is drained by lymphatics that follow the middle hemorrhoidal artery. Nodes involved from a cancer in this region can include the internal iliac nodes and the nodes of the obturator fossa. These regions deserve particular attention during the resection and irradiation of lesions in this location. When lesions occur below the dentate line, the lymphatic drainage is via the inguinal nodes and external iliac chain, which has major therapeutic implications, especially for the radiation fields. The corollary of this high risk of inguinal node involvement for the very low-lying tumors is that tumors located above the dentate line are at low risk of inguinal node involvement, and these nodes as well as the external iliacs do not need to be treated. Figure 39.13.1. Division of the rectum into upper, middle, and lower thirds. Bowel Function Fecal continence is maintained through the function of both sphincter control and the preservation of the normal muscular anatomy, which creates a neorectal angle or rectal sling. The pelvic floor is composed of the levator ani muscles, which separate the pelvis from the perineum and ischiorectal fossa. The urethra, vagina, and anus pass through the levators. Preservation of fecal continence during surgery for rectal cancer is therefore dependent on a thorough understanding of the anatomic relationships of the musculature and the sphincter mechanism. Maintenance of the sphincter apparatus without preservation of the muscular angles will not have the desired result. These anatomic constraints, especially with respect to lateral margins, make the use of adjuvant chemotherapy and radiation therapy critical to a successful surgical outcome. This is true from both an oncologic as well as a bowel function perspective. Autonomic Nerves The preservation of both bladder and sexual function is dependent on the surgeon's understanding of the autonomic nerve supply to the pelvic organs.1,2 The hypogastric plexus is formed from the sympathetic trunks as they converge over the sacral promontory. These sympathetic nerves are found beneath the pelvic P.1287 peritoneum along the lateral pelvic sidewalls lateral to the mesorectum. The second, third, and fourth sacral nerve roots give rise to parasympathetic fibers to the pelvic viscera. The parasympathetic fibers proceed laterally as the nervi erigentes to join the sympathetic fibers at the site of the pelvic plexus that is just lateral and somewhat anterior to the tips of the seminal vesicle in men.1,2 In order to preserve these structures and, therefore, sexual and bladder function, a sharp rather than a blunt technique should be used to dissect the mesorectum.3,4,5,6